Animal Parasites. 8i 



case of echinococcus cysts, bothriocephalus and teniae*). Local 

 inflammatory lesions of the skin and mucous membranes are the 

 result of mechanical injuries caused by a variety of parasites. 



The amount of nutritive substance abstracted by parasites from 

 the host is to be regarded as usually relatively insignificant. 

 Where, hov/ever, the parasites are blood-suckers, the host is placed 

 at special disadvantage ; the smallest of the round worms, if pres- 

 ent in large numbers, may cause serious loss of blood, and per- 

 haps the death of the host. The roe. the common deer and the 

 sheep, for instance, are uniformly killed by an anaemia if blood- 

 sucking strongylidas infest the rennet-stomach. Whole flocks of 

 poultry mav die out when the poultry mite, dcrmanyssus avium. 

 gets into the hennery and infests the skin of the fowls in vast 

 numbers, feeding daily upon the blood. 



A century ago our appreciation of the life history of the parasites 

 and of the diseases caused by them was but vague. It is true that most 

 of the worms found in the human alimentary canal were known to the 

 students of natural history in antiquity: and as early as the seventeenth cen- 

 tur>- itchmites and their relation with the itch were fairly well known. 

 Yet it was for the most part believed that parasites had their origin in 

 inspissated juices or pathologically altered parts of tlie human or animal 

 body by a process of spontaneous generation (gcncratio ccqutvoca). When 

 the microscope came into use and it became possible to closely observe their 

 generative organs and their products (eggs), and particularly after attempts 

 came to be made to artificially transmit the parasites by experimental feed- 

 ing of the worms and their larvc-e, the real history of their development was 

 for the first time apprehended. Redi and Swammerdam, van Doeveren and 

 Pallas, Pastor Goeze, C. Fr. Miiller, Bojanus, Abildgard, toward the end 

 of the i8th and the beginning of the iQth century, Steenstrup, v. Baer, 

 V. Siebold, van Beneden, Kiichenmeister, in the middle of the latter 

 century, corrected the mistakes and hiatuses of the earlier views and 

 investigated the most interestmg parasites ; and in recent years the studies 

 of a large number of zoologists, physicians and veterinarians (Leuckart. 

 Zenker, M. Braun, Peroncito, Grassi, Railliet, Heller. Ziirn. L. G. Neumann. 

 V. Ratz and others) have contributed to extension of our knowledge in 

 this field to such a degree that pathology and practical hygiene have been 

 decidedly benefited by their investigations. 



♦Persons handling ascaris megalocephala, the rouud worm of the horse, es- 

 pecially if engaged in mincing it, are apt to be directly affected by a conjunctivitlB 

 .-■nd lu-ticarial eruption frrmn a certain substance cxistinrr in the cu.tlcle of tho worm 

 (apparently a dermal secretion) which possesses irritative properties. 



